tcp: document tcp_max_ssthresh (Limited Slow-Start)

Base on Ilpo's patch about documenting tcp_max_ssthresh.
(see http://marc.info/?l=linux-netdev&m=117950581307310&w=2)

According to errata of RFC3742, fix the number of segments increased
during RTT time.

Just to state the occasion to use this parameter, But
about how to set parameter value, maybe some others can do it.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NShan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -280,6 +280,17 @@ tcp_max_orphans - INTEGER
 	more aggressively. Let me to remind again: each orphan eats
 	up to ~64K of unswappable memory.
 
+tcp_max_ssthresh - INTEGER
+	Limited Slow-Start for TCP with large congestion windows (cwnd) defined in
+	RFC3742. Limited slow-start is a mechanism to limit growth of the cwnd
+	on the region where cwnd is larger than tcp_max_ssthresh. TCP increases cwnd
+	by at most tcp_max_ssthresh segments, and by at least tcp_max_ssthresh/2
+	segments per RTT when the cwnd is above tcp_max_ssthresh.
+	If TCP connection increased cwnd to thousands (or tens of thousands) segments,
+	and thousands of packets were being dropped during slow-start, you can set
+	tcp_max_ssthresh to improve performance for new TCP connection.
+	Default: 0 (off)
+
 tcp_max_syn_backlog - INTEGER
 	Maximal number of remembered connection requests, which are
 	still did not receive an acknowledgment from connecting client.